Inscriptions applied to the sidewalls (tire walls) of tires often contain information of relevance for repair shops, testing organizations, gas stations and/or operators of vehicle fleets but also for tire manufacturers and tire dealers. By automatic recognition of inscriptions, valuable working time may be saved and workflow may be made more efficient.
A number of methods and devices, in particular imaging optical methods, are known for automatic recognition of inscriptions on tire walls, which may include numerals, letters and symbols. One problem is that the inscription on tire walls is usually made up of black characters on a black background, the characters being differentiable from the surface of the side wall only due to an elevation or an impression as a relief inscription. This results in a low visual contrast between the inscription and the background. Automatic recognition of the inscription by two-dimensional imaging methods is therefore difficult.
Various methods have been provided for improving the contrast: in EP 0 240 078 B1 and EP 0 294 530, the shadow casting of the relief inscription created by a directional illumination is utilized. Patent document DE 24 47 480 discusses how differences in intensity between reflected light and diffusely scattered light permit a character recognition.
U.S. Pat. Nos. 6,735,337 B2 and 6,856,697 B2 provide using the correlation with multiple partial pattern templates per character for character recognition, from which the probability of the character having been recognized correctly is derived, although perhaps not all edges of the character are visible.
A number of other approaches avoid the problem of the contrast in two-dimensional methods by using 3D methods, in particular light section methods using laser lines. Patent documents EP 1 332 334 B2, WO 03/023699 A1, DE 10 2008 023 936 A1 and U.S. Pat. No. 7,295,948 B2 discuss the segmentation and recognition of characters based on a 3D point cloud which is generated with the aid of a laser scanner in a light section method.
All the methods believed to be understood so far have in common the fact that the tire must be dismounted and rotated in a special device beneath the detection unit. These methods are therefore associated with a high labor and equipment cost.
Character recognition of inscriptions on tire walls of passing vehicles is mentioned in WO 96/10727. Patent document WO 97/07380 discusses a system which uses a 3D light section method to recognize deformations in the tire walls as they pass by. A similar procedure is discussed in EP 0 816 799. For data acquisition of a complete tire wall here, the wheel must be dismounted or the vehicle must be standing on a roller type test stand or the recording system must be mounted on the vehicle, so that it moves with the vehicle.